Systems and methods of blood-based therapies having a microfluidic membraneless exchange device

ABSTRACT

The present invention is directed to devices, systems and methods for removing undesirable materials from a sample fluid by contact with a second fluid. The sample fluid flows as a thin layer adjacent to, or between, concurrently flowing layers of the second fluid, without an intervening membrane. In various embodiments, a secondary separator is used to restrict the removal of desirable substances and effect the removal of undesirable substances from blood. The invention is useful in a variety of situations where a sample fluid is to be purified via a diffusion mechanism against an extractor fluid. Moreover, the invention may be used for the removal of components from a sample fluid that vary in size. When blood is the sample fluid, for example, this may include the removal of ‘small’ molecules, ‘middle’ molecules, macromolecules, macromolecular aggregates, and cells, from the blood sample to the extractor fluid.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of and claims the benefit ofU.S. Ser. No. 10/801,366, filed Mar. 15, 2004 and a continuation-in-partof PCT International Application No. PCT/US04/07966, filed Mar. 15,2004, and claims the benefit of U.S. Provisional Patent Application No.60/454,579, filed Mar. 14, 2003, each of which is hereby incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

Generally speaking, the present invention relates to the purification ofa sample fluid. More particularly, the present invention relates to thepurification of a sample fluid (e.g., blood fluid) by selectivelyremoving components using a microfluidic membraneless exchange device.

BACKGROUND OF THE INVENTION

Extracorporeal processing of blood is known to have many uses. Suchprocessing may be used, for example, to provide treatment of a disease.Hemodialysis is the most commonly employed form of extracorporealprocessing for this purpose. Additional uses for extracorporealprocessing include extracting blood components useful in either treatingothers or in research. Apheresis of plasma (i.e., plasmapheresis) andthrombocytes, or platelets, are the procedures most commonly employedfor this purpose.

Many different extracorporeal blood processing processes have beendeveloped, each of which seeks to remove certain components from theblood, depending on the reason for processing the blood. (It will beunderstood that as used herein, blood, or blood fluid, refers to anyfluid having blood components from which extraction of certaincomponents, such as toxins or metabolites present in excess, isdesired.) The most common process utilizes an artificial membrane ofsubstantial area, across which selected blood components are induced toflow. This flow is generally induced by a transmembrane difference ineither concentration or pressure, or a combination of the two. Anotherform of blood processing calls for the separation of certain componentsfrom blood by passing the blood over sorbent particles. In yet otherforms of blood processing, not practiced as commonly, blood is directlycontacted with an immiscible liquid (e.g., a fluorocarbon liquid), withthe desired result being the removal of dissolved carbon dioxide and theprovision of oxygen. The usefulness of blood processing techniquesemploying immiscible liquids is limited, however, because theseimmiscible liquids generally have very limited capacity to accept theblood components that it is desirable to extract.

One common example of a therapeutic use for blood processing is themitigation of the species and volume imbalances accompanying end-stagerenal disease. The population of patients treated in this manner (i.e.,through hemodialysis) exceeds 260,000 and continues to grow, with thecost of basic therapy exceeding $5 billion per year excludingcomplications. The overwhelming majority of these patients (about 90%),moreover, are treated in dialysis centers, generally in thrice-weeklysessions. While procedures have been—and continue to be—refined, thecomponents and the geometry of hemodialysis were largely fixed in the1970's: a bundle of several thousand, permeable hollow fibers, eachabout 25 cm long and about 200 μm internal diameter, perfused externallyby dialyzing solution, with the device operated principally in adiffusive mode but with a transmembrane pressure applied to induce aconvective outflow of water. Upward of 120 liters per week of patientblood are dialyzed against upwards of 200 liters per week of dialyzingsolution, often in three weekly treatments that total as little as sevento nine hours per week. These numbers vary somewhat, and competingtechnologies exist, but the basic approach just described predominates.

Despite the benefits of therapies (e.g., hemodialysis) using the variousforms of blood processing described above, the prolongation of lifeachieved is complicated by the progression and complexity of the diseasethe therapies are used to treat (few patients on dialysis are evercompletely rehabilitated), and by several problems that are innate tothe therapies themselves. For example, problems arise with bloodprocessing as a result of the contact of blood with extensive areas ofartificial membrane (as in the case of hemodialysis), and well as thecontact of blood with sorbents or immiscible fluids as described above.In particular, this contact often induces biochemical reactions in theblood being processed, including the reactions that are responsible forclotting, activation of the complement system, and irreversibleaggregation of blood proteins and cells.

Another problem associated with known blood processing techniques isthat the contact of blood with an artificial membrane (or anothermedium, such as a surface of a sorbent or immiscible fluid) is likely tocause the blood-medium interface to become fouled. It is generally knownthat therapeutic interventions (e.g., those related to end-stage renaldisease) are optimally conducted with slow delivery and in as nearly acontinuous fashion as possible, in emulation of the continuous action ofa natural kidney. However, fouling caused by the contact of blood withthe medium limits the time that a device which contains these interfacescan be usefully employed. As a result, portable blood processing devicesbecome impractical, and patients are generally forced to undergo thetype of episodic dialysis schedule described above, which creates manynegative side effects such as physical exhaustion and excessive thirst.Moreover, even while daily dialysis (e.g., 1.5-2.0 hours, six days perweek) or nocturnal dialysis (e.g., 8-10 hours, 6-7 nights per week)improves this situation by extending treatment times, a patient usingone of these forms of treatment or a partner is required to mastertechnical procedures and, as many find especially onerous, to accesspatient blood by the insertion of usually two relatively large needlesinto a vein or artificial, subcutaneous fistula.

In light of the above, it would be desirable to provide techniques forprocessing blood in which treatment times are extended (withconsequently lower rates of flow) and that do not require a patient toinitiate and terminate blood access. Moreover, it would also bedesirable to provide techniques for processing blood that eliminate (orat least reduce) the inducement of undesirable biochemical reactions,and where the blood-medium interfaces do not become fouled.

SUMMARY OF THE INVENTION

The above and other deficiencies associated with existing bloodprocessing processes are overcome in accordance with the principles ofthe present invention which are described below. According to one aspectof the invention, a membraneless exchange device for extractingcomponents from a sample fluid is described which includes first, secondand third inlet channels, first, second and third exit channels and amicrofluidic extraction channel connected to the first, second and thirdinlet channels and the first, second and third exit channels and a flushport. Moreover, laminar flows of a first extractor fluid, the samplefluid, and a second extractor fluid are established inside theextraction channel, and sheathing of the sample fluid by the first andsecond extractor fluids substantially limits contact between the samplefluid and the surfaces of the extraction channel.

According to another embodiment of the present invention, a system forperforming hemodialysis is provided which includes a membranelessexchange device including first and second dialysate inlet channels,blood inlet and exit channels, first and second dialysate exit channelsand a microfluidic dialysis channel connected to the first and seconddialysate inlet and outlet channels and the blood inlet and exitchannels. Moreover, laminar flows of a first dialysate fluid, bloodfluid, and a second dialysate fluid are established in order inside thedialysis channel, and at least some of the components of the blood fluidexit the device through the first and second dialysate exit channels.Additionally, according to the invention, a secondary processor receivesthe dialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels.

In yet another embodiment of the present invention, a method forextracting components from a sample fluid is provided which includesestablishing laminar flows of a first extractor fluid, the sample fluidand a second extractor fluid inside a microfluidic extraction channel.Sheathing of the sample fluid by the first and second extractor fluids,moreover, substantially limits contact between the sample fluid and thesurfaces of the extraction channel. The method further includeswithdrawing the first extractor fluid, the sample fluid and the secondextractor fluid from the extraction channel such that at least a portionof the sample fluid is removed together with the first extractor fluidand the second extractor fluid and apart from the remainder of thesample fluid.

A method for performing hemodialysis is also provided which includesestablishing laminar flows of a first dialysate fluid, blood fluid and asecond dialysate fluid inside a microfluidic extraction channel,withdrawing the first dialysate fluid, the blood fluid and the seconddialysate fluid from the extraction channel such that at least some ofthe components of the blood fluid are removed together with the firstdialysate fluid and the second dialysate fluid and apart from theremainder of the blood fluid, and providing the first and seconddialysate fluids and the at least some of the components of the bloodfluid to a secondary processor.

In general, however, the present invention is directed towardmicrofluidic membraneless exchange devices and systems, and methods ofmaking the same, for selectively removing undesirable materials from asample fluid (e.g., blood fluid) by contact with a miscible fluid(extractor fluid or secondary fluid, e.g., dialysate). A microfluidicdevice, as considered in this application, has channels whose height isless than about 0.6 mm, where “height” is the dimension perpendicular tothe direction of flow and also perpendicular to the interfacial areaacross which transport occurs. For example, flow patterns and speciesexchanges occur when blood is flowed as a thin layer adjacent to, orbetween, concurrently flowing layers of a secondary fluid, without anintervening membrane. The secondary fluid, moreover, is generallymiscible with blood and diffusive and convective transport of allcomponents is expected. The following reference which refers tomembraneless devices described below is hereby incorporated by referencein its entirety: Leonard et al., Dialysis without Membranes: How andWhy?, Blood Purification 22 (1) 2004 92-100.

Sheathing a core of blood with the miscible fluid, or assuring that themiscible fluid lies between at least a substantial portion of the bloodand the enclosing boundaries of the flow path, prevents or at leastlimits contact of the blood with these boundaries. In turn, thisconfiguration of the two fluids prevents or at least reduces theundesirable activation of factors in the blood, thereby minimizingbioincompatibilities that have been problematic in prior techniques ofblood processing.

The invention also eliminates or at least substantially reduces thefouling reactions that have been known to be a major deterrent to thecontinuous use of an extracorporeal extraction device. In particular, asthe primary transport surface in the membraneless exchange device (alsoreferred to herein as a membraneless separator) of the invention isintrinsically non-fouling, a major deterrent to long-term or continuousoperation is removed, opening the possibility to the design andconstruction of small, wearable devices or systems with the recognizedbenefits of nearly continuous blood treatment. Such a device or systemcould be very small and worn or carried by the patient (e.g., outside ofa hospital or clinic setting), and could be supplied with externalbuffer reservoirs (in a back-pack, briefcase, or from a reservoirlocated in the home, located at the place of work, etc.). Further,because fouling would be reduced, and sustained operation at low bloodflows over long times would be allowed, such anticoagulation as might berequired is likely to have an effect confined to the extracorporealcircuit. As understood by those skilled in the art, avoiding systemicanticoagulation outside of the clinic is highly desirable.

The devices, systems and methods of the invention described herein alsohave the benefit of being capable of diffusing various blood componentshaving different sizes. In particular, the flow of blood and a misciblefluid with which it is in contact can be controlled for the purpose ofachieving the desired separation of components. For example, flowadjustment can minimize cellular migration across the interface. Asheath fluid can be used to give the discrimination of a membranebetween large and small molecules that cannot be achieved by a denudedinterface, no matter how exposure time (adjacent flows) is varied. Forexample, as explained below, various flow conditions may be used thatcause blood cells to move away from the blood-liquid interface, therebymaking it is possible to “skim” blood in order to remove substantialamounts of plasma, without cells.

As also discussed below, membraneless contact of a thin layer of bloodwith a sheathing fluid according to the present invention may be used tocause high rates of exchange per unit area of blood-sheathing fluidcontact for all solutes, but with a discrimination among free (unbound)solutes that is less than the square-root of the ratio of theirdiffusion coefficients. Moreover, while high exchange rates (e.g., oftoxic substances) are often desirable, indiscriminate transport is not.Therefore, according to the principles of the present invention, amembraneless exchange device as described herein is used in conjunctionwith at least one secondary processor (e.g., a membrane device or othertype of separator) in order to restrict the removal of desirablesubstances and effect the removal of undesirable substances from blood.The efficiency of such a secondary separator is greatly increased by theuse of a primary separator that is capable of delivering cell-depleted(or cell-free) fractions of blood to it. Therefore, according to anotheraspect of this invention, transport of molecular components of blood tothe sheathing fluid may be indiscriminate. The sheathing fluid, carryingboth molecular components which are, and those which are not, desirableto remove from blood, is provided to the secondary separator, such thatthe fluid entering the secondary separator is substantially cell-free.The secondary separator, meanwhile, regulates the operation of themembraneless separator through the composition of the recycle streamthat it returns (directly or indirectly) to the sheath fluid inlets ofthe membraneless separator. According to the principles of the presentinvention, moreover, a membrane-based secondary separator used in thismanner is able to achieve much higher separation rates becauseconcentration polarization (i.e., the accumulation of material rejectedby the secondary separator on the upstream side of the separator) islimited to proteins and does not involve cells. Moreover, because cellswould be retained in the primary separator (i.e., the membranelessexchange device), they would see artificial material only on its conduitsurfaces, not on its liquid-liquid contact area, whencebioincompatibilities should be much reduced. As such, it should beunderstood that the need for anticoagulation may be greatly reduced oreliminated.

There is a need to establish and break extracorporeal blood flow. Inplain terms, patients dislike the needle sticks and even the needlewithdrawals associated with home dialysis and which are necessitated byany dialysis system whose sterile parts are not small enough to travelwith or within the patient. While the present invention still requires aconnect/disconnect, it is done at the washing fluid interface. Theextracorporeal blood flow is continuous and because blood sees nomembrane it can function indefinitely without need to interrupt theblood flow. Further features of the invention, its nature and variousadvantages, will be more apparent upon consideration of the followingdetailed description, taken in conjunction with the accompanyingdrawings, in which like reference characters refer to like partsthroughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the velocity profile of a core stream of blood sheathed onboth of its sides by a dialysate fluid calculated for blood with aviscosity assumed twice that of the dialysate fluid and with acenterline velocity of 5 cm/sec.

FIG. 2 shows a plot using Loschmidt's formula of 1870, where each fluidlayer has the same thickness.

FIG. 3 shows a simplified view of a membraneless separator constructedin accordance with the principles of the present invention.

FIG. 4 shows a membraneless separator used for the purpose ofplasmapheresis in accordance with the principles of the presentinvention.

FIG. 5 shows the image of a portion of the membraneless separator ofFIG. 5 while plasma is being skimmed from blood, as obtained by using aCCD camera.

FIG. 6 shows a simplified block diagram of a system including amembraneless separator and a secondary separator in accordance with theprinciples of the present invention.

FIG. 7 shows a more detailed view of a system including primary andsecondary separators in accordance with the principles of the presentinvention.

FIG. 8 shows the configuration of a system subdivided into three unitsin accordance with the principles of the present invention.

FIG. 9 shows the routing of fluids between separate units in accordancewith the principles of the present invention.

FIG. 10 is a diagram showing a cross-section of blood flowing at a meanvelocity of 68.34 mm/sec, sandwiched between two layers of sheath fluid,each flowing at a mean velocity of 37.52 mm/sec. Each layer is flowingfrom left to right. Each layer is 100 micrometers (0.1 mm) thick. Theblood layer is isolated from wall contact by the layers of moving sheathfluid and experiences a very low rate of shear, evidenced by its veryflat velocity profile.

FIG. 11 is a graph showing Boltzmann's relationship expressed asextraction fraction, E, between two stagnant solute layers, each ofthickness, B, as a function of time, t [18, 19]. The solute has adiffusion coefficient of value D. In the present paper we apply thisresult to a different but related situation: A central blood layer ofthickness 2B diffuses into two sheathing streams, also of thickness, 2B.Each half of the blood layer feeds one of the two sheathing streams. Theapproximate calculations reported here assume that each half of theblood layer, whose thickness is B, equilibrates with the sheathinglayer, whose thickness is 2B, but whose velocity is half that of theblood, and—notwithstanding the operational differences—behaves accordingto the Boltzmann result. Thus a 100 μm blood layer is treated as two 50μM layers, each communicating with a 50 μm layer of sheathing fluid thatis traveling with the same velocity as the blood. Urea extraction is 90%of its maximum value for equal flows when E equals 0.45. Thiscorresponds to a value Dt/B² of 0.848 and requires, for molecules whosediffusion coefficients are in the neighborhood of 10⁻⁵ cm²/sec a bloodresidence time in the contact area of 2.1 sec. The approximation hasbeen validated with more precise finite-element calculations and isaccurate within 5%.

FIG. 12 is a diagram showing how blood and sheath fluid circulatetogether through the blood-sheath contractor, driven by a 2-headedperistaltic pump. The flush port is shown. Blood returns to patient.Sheath fluid enters membrane dialyzer and is then recirculated to theblood-sheath contactor. Dialysate, when connected, flows countercurrentto sheath fluid in a membrane dialyzer. Blood and sheath fluid flow atall times. Dialysis is effective when dialysate is connected. With adevice to maintain trans-membrane pressure in the dialyzer,ultrafiltration can occur in the absence of dialysate. The flush portallows for periodic injection of sterile saline into the sheath streamin order to force cells back into bloodstream. Of course, it is likelynot possible to keep all cells out of the sheath fluid. The removal ofcells that find their way into the sheath fluid is more complicated, andless desirable than returning the cells to the bloodstream. In the casewhere the cells accumulate to concentrations in the sheath fluid thatare undesirable, a periodic flush can be utilitzed. The volume of theflush need only be a small fraction of all fluid removed by thesecondary separator. The flush allows for effective return of thesecells to the bloodstream, and clarification of the sheath fluid.

FIG. 13 is a photograph of a prototype of a miniaturized dialyzer. Fiberlength is about 9 cm, with total surface area in the unit shown of 500cm² of polysulfone hollow fiber. (Manufacturer: Saxonia Biotec,Radeberg, Germany.)

FIG. 14 is a schematic drawing of a wrist-size wearable dialysis system.The blood-sheath fluid contactor, shaped as a plate about 5.5×8 cm issized to lie on the forearm. A two-headed peristaltic pump, dialyzer,and control module are mounted above the plate. Power for the pump canbe provided by a battery shaped to match the dimensions of thecontactor. The assembly would be placed under a smooth cover in actualpatient use.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a membraneless exchange devicefor extracting components from a sample fluid is described whichincludes first, second and third inlet channels, first, second and thirdexit channels and a microfluidic extraction channel connected to thefirst, second and third inlet channels and the first, second and thirdexit channels, and which includes a flush port. One embodiment of theflush port is depicted in FIG. 12. The flush port is capable of ‘reverseflushing’. In usage of the device of the invention, there may be cellsentering the sheath fluid and this is undesirable. If no correction isapplied (some method of removing the cells from the sheath fluid, orsome method of returning the cells to the blood), it is inevitable thatthe composition of the sheath fluid, over a long enough period of time,will approach that of blood, thus obviating many if not all of theadvantages of the invention. While there are many ways of refreshing thesheath fluid (such as filtration or simply discarding and replacing it)it is preferable that the cells that had entered the sheath fluid beperiodically returned to the blood. If there has been active removal inthe sheath circuit of toxins and superfluous metabolites, the quantityof these materials returned to the patient along with the cells will benegligible and valuable cells will not be lost. It will be appreciatedthat cells entering the sheath fluid have only these possible fates: (1)to be destroyed, which is generally undesirable unless the products ofthe destruction are removed, since these products may be harmful ifreturned to the patient, (2) to be removed from the system byaccumulation on a filter, which presents numerous technicaldifficulties, or (3) to be returned to the blood stream. The latter fateis the most desirable. If it is, however, achieved only when the sheathfluid reaches the same cell concentration as blood, much of the benefitof providing the sheath will be lost. Thus, the benefits of sheathingare preserved by periodically forcing the return of cells by the simpleflushing process described. So long as the volume of sheath fluid andthe substances contained in it are small compared with the volume offluid removed since the last flush, any interruption of the removalprocess is inconsequential. For example, one embodiment of the inventionprovides water extraction from blood at a rate of 3 ml/min. In thisembodiment, the total volume of sheath fluid is approximately 3-5 ml. Ifthis fluid is displaced by fresh, sterile saline, it will be necessaryto remove that much additional fluid from the system, but that can beaccomplished in less than 2 minutes. In one embodiment of the invention,flushing will be carried out about not more than once an hour. In otherembodiments, the flushing will be carried out about twice an hour, aboutonce every two hours, about once every 2.5 hours, about once every 3hours, about once every 3.5 hours, about once every 4 hours, about onceevery 4.5 hours, about once every 5 hours, or less often. Thus, in thisembodiment, the time of treatment necessary to achieve a given amount ofremoval of toxins and surplus metabolites, including water, will beincreased by not more than 3%.

The membraneless device of the invention utilizes laminar flows withinthe device. Laminar flows of a first extractor fluid, the sample fluid,and a second extractor fluid are established inside the extractionchannel, and sheathing of the sample fluid by the first and secondextractor fluids substantially limits contact between the sample fluidand the surfaces of the extraction channel. In one embodiment of thedevice, at least 90% of the sample fluid is sheathed by the first andsecond extractor fluids. In other embodiments, 95% of the sample fluidis sheathed. In yet other embodiments, at least a portion of the samplefluid exits the device with the first extractor fluid through the firstexit channel, and advective transport of molecules within saidextraction channel is substantially nonexistent. The composition of thefirst extractor fluid, moreover, is substantially the same as thecomposition of the second extractor fluid is various embodiments. Inother preferred embodiments, the sample fluid flow is between the firstand second extractor fluid flows. Moreover, a first diverter is formedfrom a portion of the first exit channel and a portion of the secondexit channel, while a second diverter is formed from a portion of thesecond exit channel and a portion of the third exit channel. It shouldalso be understood that the device may include a first interface formedbetween the first extractor fluid flow and the sample fluid flow that isaligned with at least a portion of the first diverter, and may alsoinclude a second interface formed between the second extractor fluidflow and the sample fluid flow that is aligned with at least a portionof the second diverter. In various embodiments of the invention,moreover, the sample fluid is blood fluid, in which case it iscontemplated that the components extracted from the sample fluid arenon-cellular components of the blood fluid. Additionally, the device mayuse a first pump for controlling the flow of extractor fluid in theextraction channel, and may use a second pump for controlling the flowof sample fluid in the extraction channel. When a first pump is used, itmay be an injection pump that controls the flow of extractor fluid intothe extraction channel, and a withdrawal pump may be used that controlsthe flow of extractor fluid out of the extraction channel. In oneembodiment, one pump controls three (3) streams (or five, if the twoextractor fluids are considered to be separate). In such a case, theflow rate of the 6th stream is then determined by physics. In this case,the sixth stream is the existing blood from the patient. In this case,therefore, the fraction of the blood volume that is removed iscontrolled by accurately setting the other pumps. Anything else, absenta membrane, does not give the control that is needed. In one embodiment,the pump used should control differentially the flow extraction in andout of the device so that the forces are controlled and thus prescribesthe volume of fluid taken from the blood stream.

In various embodiments, additionally, a source of extractor fluid isconnected to said first inlet channel and a source of sample fluidconnected to said second inlet channel. It will be understood that thesource of sample fluid can be, for example, a human being. In preferredembodiments, moreover, the extraction channel of the device according tothe invention has a height of less than 600 μm, and has awidth-to-height ratio of at least ten. The device may also be used in asystem for extracting components from a sample fluid, where the systemalso includes a secondary processor that receives the first extractorfluid, the second extractor fluid and at least some of the components ofthe sample fluid upon exiting the extraction channel. It will beunderstood that the secondary processor may be, for example, a membranedevice or a sorption device or a reactor capable of transformingcomponents of the sample fluid.

According to another embodiment of the present invention, a system forperforming hemodialysis is provided which includes a membranelessexchange device including first and second dialysate inlet channels,blood inlet and exit channels, first and second dialysate exit channelsand a microfluidic dialysis channel connected to the first and seconddialysate inlet and outlet channels and the blood inlet and exitchannels. Moreover, laminar flows of a first dialysate fluid, bloodfluid, and a second dialysate fluid are established in order inside thedialysis channel, and at least some of the components of the blood fluidexit the device through the first and second dialysate exit channels.Additionally, according to the invention, a secondary processor receivesthe dialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels. In various embodiments, the secondary processor filters thedialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels, and returns the filtered fluid to the first and seconddialysate inlet channels. In certain preferred embodiments, thesecomponents of the blood fluid are substantially non-cellular componentsof the blood fluid. In other embodiments, sheathing of the blood fluidby the first and second dialysate fluids substantially limits contactbetween the blood fluid and the surfaces of the dialysis channel.Moreover, the secondary processor may be a membrane device, or may be asorption device, for example. It will also be understood that thecomposition of the first dialysis fluid may be substantially the same asthe composition of the second dialysis fluid. According to other aspectsof the invention, meanwhile, a first diverter is formed from a portionof the first dialysate exit channel and a portion of the blood exitchannel, and a second diverter is formed from a portion of the bloodexit channel and a portion of the second dialysate exit channel. A firstpump for controlling the flow of dialysate fluid in the dialysis channeland a second pump for controlling the flow of blood fluid in thedialysis channel may also be used in accordance with the principles ofthe present invention. According to several embodiments, the interfacebetween the first dialysate fluid and the blood fluid is varied byadjusting the volumetric flow rates of the first dialysate fluid and theblood fluid. In other embodiments, the interface between the blood fluidand the second dialysate fluid is varied by adjusting the volumetricflow rates of the blood fluid and the second dialysate fluid.Additionally, a detector for detecting a presence of an undesired bloodcomponent within the dialysate fluid upon exiting the dialysis chambermay be used. In this case, for example, the detector is a photodetector. According to another aspect of the invention, a first pump forcontrolling the flow of dialysate fluid in the dialysis channel isadjusted based on said detected presence of an undesired blood componentwithin said dialysate fluid. Moreover, for example, the velocities ofthe laminar flows of the first dialysate fluid, the blood fluid and thesecond dialysate fluid are adjusted based on the detected presence of anundesired blood component within the first and second dialysate fluidsaccording to the invention. Additionally, according to the invention,the first and second dialysate fluids may include at least one of thefollowing: a hyper osmolar solution, a solution high in glucose content,or a polyelectrolye osmotic agent.

In yet another embodiment of the present invention, a method forextracting components from a sample fluid is provides which includesestablishing laminar flows of a first extractor fluid, the sample fluidand a second extractor fluid inside a microfluidic extraction channel.Sheathing of the sample fluid by the first and second extractor fluids,moreover, substantially limits contact between the sample fluid and thesurfaces of the extraction channel. The method further includeswithdrawing the first extractor fluid, the sample fluid and the secondextractor fluid from the extraction channel such that at least a portionof the sample fluid is removed together with the first extractor fluidand the second extractor fluid and apart from the remainder of thesample fluid. According to the invention, moreover, establishing laminarflows includes providing first, second and third inlet channels andproviding first, second and third exit channels. Additionally, forexample, the method includes providing the first and second extractorfluids and the at least a portion of the sample fluid to a secondaryprocessor.

A method for performing hemodialysis is also provided which includesestablishing laminar flows of a first dialysate fluid, blood fluid and asecond dialysate fluid inside a microfluidic extraction channel,withdrawing the first dialysate fluid, the blood fluid and the seconddialysate fluid from the extraction channel such that at least some ofthe components of the blood fluid are removed together with the firstdialysate fluid and the second dialysate fluid and apart from theremainder of the blood fluid, and providing the first and seconddialysate fluids and the at least some of the components of the bloodfluid to a secondary processor. In various embodiments, the method alsoincludes using the secondary processor to filter the first and seconddialysate fluids and the at least some of the components of the bloodfluid, as well as returning the filtered fluid from the secondaryprocessor to the extraction channel. In yet other embodiments, themethod includes sheathing the blood fluid by the first and seconddialysate fluids to substantially limit the contact between the bloodfluid and the surfaces of the dialysis channel.

Referring to FIG. 1, calculated for blood with a viscosity assumed twicethat of the sheathing fluid and with a centerline velocity of 5 cm/sec,a flow path length of 10 cm would result in a contact time of slightlylonger than 2 sec. The steady contact of two moving liquids for anexposure time determined by the length of their contact area divided bytheir interfacial velocity (τ=L/ν) is highly analogous to the suddenexposure of one volume of stagnant fluid to another for a specifiedtime. Thus, what happens to the flowing fluids along their shared flowpath is comparable to what would happen to two stagnant fluids overtheir exposure time to each other. The stagnant fluid problem was solvedby Loschmidt in 1870.

$E = {\frac{1}{2} - {\frac{4}{\pi^{2}}{\overset{\infty}{\sum\limits_{0}}{\frac{1}{( {{2n} + 1} )^{2}}{\exp \lbrack {{- ( {{2n} + 1} )^{2}}( \frac{\pi}{2B} )^{2}{Dt}} \rbrack}}}}}$

for which the zeroth order term,

${E = {\frac{1}{2} - {\frac{4}{\pi^{2}}{\exp ( {{- \lbrack \frac{\pi}{2B} \rbrack^{2}}{Dt}} )}}}},$

suffices when

${( \frac{\pi}{2B} )^{2}{Dt}} > {0.7.}$

This formula greatly simplifies the estimation of how much mass can betransferred between fluids in a membraneless system. In particular, thisformula provides an approximation of the extraction E of a componentwith a diffusion coefficient D when two liquids flow side-by-side andremain in contact for an interval of time, t.

FIG. 2, meanwhile, shows a plot using a version of Loschmidt's formula,where each fluid layer has the same thickness B (i.e., B is thehalf-thickness of the sheathed layer of sample fluid). The situationshown in the plot of FIG. 2 can be interpreted as a blood layer, ofthickness B, contacting a layer of sheathing fluid (i.e., extractorfluid). The sheathing layer is presumed to be at zero concentration andE is the fraction of material in the blood layer that is extracted in atime t, where D is the diffusion coefficient of the extracted substance.If a layer of thickness twice B is bounded on both sides by fluid layersof thickness B, the formula still applies, as written. As indicated bythis formula, E cannot exceed ½ since the prescription of concurrentflow allows, at best, the two fluids to come to equilibrium.

For example, if one prescribes 90% of maximum extraction (E=0.45), theratio Dt/B² must be approximately 0.86. Any combination of diffusivity,layer thickness, and exposure time that produces this value will producethe same extraction. Moreover, it can be shown that the necessary area(2LW) to achieve this extraction equals 0.86 BQ/D, where Q is the blood(and sheath fluid) flow rate. Thus, for urea (D=10⁻⁵ cm²/sec) at a bloodflow rate of 0.3 cm³/sec, the required area is 2.57 B 10 ⁴ cm². If B istaken to be 100 μm, the required area is 257 cm². This flow correspondsto what might be needed in a wearable artificial kidney. If, instead, aconventional flow of 5 cm³/sec were used, the required area would be4300 cm². Thinner films, moreover, would require less area but wouldresult in higher shear rates and pressure gradients. In terms ofextraction, any combination of length L and width W that produces therequisite area is equivalent. (If one assumes D for albumin to be 5 10⁻⁷cm²/sec, its extraction would be 0.116, 26% of that for urea,unchangeable at this extraction level for urea).

It should be noted that use of the Loschmidt formula with flowingsystems introduces an incongruity that prevents precise estimation ofmass transfer rates and clearances, given that it presumes that bothfluids are moving at uniform velocity. In particular, it provides anexcellent approximation for the sheathed fluid (blood), but ignores thenearly linear decay in velocity with distance from the interface in thesheathing fluid. Nevertheless, the Loschmidt formula is adequate fordesign purposes when the sheathing layer has a total thickness (2B) thatis twice that of its half of the blood layer (as shown in FIG. 1), andthus a rate of flow nearly equal to its half of the central stream.

The shear-induced self-diffusion coefficient of cells, meanwhile, can beestimated by using the expression of Leighton and Acrivos (1987) forconcentrated suspensions: D_(particle)∝φ²a²{dot over (γ)}², where φ isthe particle volume fraction, α is the particle radius, and {dot over(γ)} is the shear rate. Then, the characteristic displacement of a cellcan be expressed as Δy∝√{square root over (D_(particle)t)}. Choosingrepresentative values for the layered flow system such that the cellvolume fraction φ≅0.45/2=0.225, the average radius a of the red bloodcell≅2.5 μm, and the average shear rate {dot over (γ)} over the bloodlayer≅3 to 28 s⁻¹ (based on an average velocity range of 0.5 to 5 cm/s),we calculate that D_(particle)˜10⁻⁸ cm²/s, which is approximately threeorders of magnitude smaller than the typical diffusion coefficient ofsmall solutes. Based on this value of the shear-induced diffusioncoefficient (and assuming 10 sec of contact between layers), it isestimated that blood cells are displaced by a characteristic distanceΔy≅3 τo 9 μM from the central layer, depending on the choice of bloodvelocity and the concomitant shear rate. As explained in greater detailbelow, this low distance of cell migration away from the central layerfacilitates the removal of cell-free portions of blood by themembraneless separators described herein.

It should be noted that, according to one aspect of the presentinvention, the removing of undesirable materials from a sample fluidoccurs under conditions that prevent advective mixing of blood and thesecondary fluid. In its general usage herein, advection is used todescribe the transport of fluid elements from one region to another, andis used to distinguish disordered convection from diffusion unaided byconvection or diffusion in the presence of only ordered andunidirectional convection. The term advection is therefore used to meana form of transport within a fluid or between two contacting misciblefluid in which clumps of fluid from two different positions areeffectively interchanged. Advection, so defined, can occur in turbulentflows or in unstable laminar flows. Advective mixing, moreover, is oftenpurposefully induced by the application of a moving agitator blade to afluid. The prevention of advective mixing and the short contact timesthat lead to small areas of contact (and, in turn, to a small devicethat has a small size and a limited extracorporeal blood volume) isgreatly facilitated by the use of a microfluidic geometry. An increasein channel height raises requisite contact time and tends to reduce thestability of the sheathed flow. When total blood layer thickness is 25,50, or 100 μm, and the blood flow is 20 ml/min (as it might be with awearable artificial kidney), the interfacial area needed to cause asubstance such as urea (D=10⁻⁵ cm²/sec) to reach 90% of equilibrium is,respectively, 18, 36, and 71 cm².

As mentioned above, the devices, systems and methods of the presentinvention allow the purification of blood without the use of a membraneby contact of the blood with a miscible fluid under conditions thatprevent advective mixing. It will be clear from the detailed descriptionof various embodiments of the invention provided below that theinvention is useful in hemodialysis, for example. However, it shouldalso be noted, and understood by those skilled in the art, that thepresent invention is also useful in other situations where a samplefluid is to be purified via a diffusion mechanism against another fluid(e.g., an extractor fluid).

According to the principles of the present invention, the purificationtechniques described herein enable the flow of blood, completely orpartially surrounded by another liquid (e.g., extractor fluid) such thatthe streams are contacted in a small channel and are subsequentlyseparated at the end of the channel. The middle stream is, thus, theblood to be purified, while the surrounding stream (or streams) is theextractor fluid. This membraneless contact, or sheathing of blood withlayers of a miscible fluid, according to principles of the presentinvention, may occur along a flow path whose cross-section is eitherrectangular, preferably of great breadth and limited thickness, orcircular. The invention is not limited in this manner.

Persons skilled in the art will appreciate that the requisite transportareas, moreover, can be achieved by combinations of channel length,width, and number according to the principles of the present invention.In particular, Area=2 (top and bottom)×width×length×number of channelsstacked or otherwise arrayed in parallel. (As used herein, the term“width” refers to a dimension perpendicular to the direction of flow andparallel to the interface between the two liquids, while, as explainedabove, the term “height” refers to a dimension perpendicular to thedirection of flow and also perpendicular to the interface between thetwo fluids). It is shown herein that the competing requirements of smallheight (to avoid excessive diffusion times and in-process volumes),short length (to avoid excessive pressure drop) and practicallimitations on width of a single device, which suggests the need toarray them in parallel, side-by-side or in a stack can be satisfied inpractical microfluidic devices.

FIG. 3 shows a simplified view of a membraneless separator 300fabricated in flat-sheet configuration in accordance with the principlesof the present invention. According to one embodiment of the presentinvention, three flat strips of copper foil, each three centimeterswide, four centimeters long and 100 microns thick, are soldered in theirmid-sections to form extraction channel 302. The ends (one centimeter)of the outer pieces are bent 30 degrees outward to form three separateinlet channels 304, 306 and 308 and three corresponding exit channels310, 312 and 314 as shown in FIG. 3. According to the invention, thepieces are then coated with a mold release agent, and the channel isthen placed in a Petri dish. At this time, an amount of PDMSprecursor/curing agent mixture (10:1 ratio), sufficient to form a twocentimeter-thick polymer layer after curing, is poured into the dish.After curing, the foil assembly is easily released from the PDMSreplica, and the replica is sandwiched between two partially cured flatpieces of PDMS and annealed to form a well-sealed channel. Finally,slight vacuum is applied during the annealing to remove air bubblestrapped between the flow channel module and the flat pieces, and thesealed separator 300 is then ready for use (preferably after the chip isrinsed with ethanol and with de-ionized water, and then dried withcompressed nitrogen gas). A flat piece of PDMS which served as a coverto seal the chip by adhesion is also preferably cleaned and dried in thesame manner.

It will be understood that the particular fabrication process describedabove is for purposes of illustration only. For example, the dimensionsof membraneless separator 300 may be altered without departing from thescope of the present invention. Additionally, for example, it will beunderstood that other fabrication processes not described may also beemployed.

FIG. 4 shows a membraneless separator 400 according to the principles ofthe present invention. Similar to separator 300 described above,separator 400 includes an extraction channel 402, three separate inletchannels 404, 406 and 408 and three corresponding exit channels 410, 412and 414. As also shown in FIG. 4, a first diverter 416 is formed fromportions of exit channels 410 and 412, while a second diverter 418 isformed from portions of exit channels 412 and 414. It will beunderstood, however, that the invention is not limited by the number ofexit channels (or inlet channels) that are used, nor is the inventionlimited by the number of diverters formed therefrom.

As illustrated in FIG. 4, membraneless separator 400 can be used as aplasmapheresis device in accordance with the principles of the presentinvention. For example, as shown in FIG. 4, plasma from the bloodentering extraction channel 402 through inlet channel 406 is skimmed andexits with sheath fluid through exit channels 410 and 414. This processof skimming is explained in greater detail below in connection with FIG.7. In the case of plasmapheresis, the secondary separator is not neededsince the product can be taken directly from the sheath fluid. However,not all of the sheath fluid can be taken, since a substantial fractionmust be recirculated to the input of the primary separator.

FIG. 5, meanwhile, shows an image of the right-most portion of separator400 shown in FIG. 4, as obtained by using a CCD camera (Sensys0401E,Roper Scientific). In particular, the image of FIG. 5 illustrates plasmabeing skimmed from blood according to the principles of the presentinvention. As shown in FIG. 5, a portion of the blood 501 providedthrough inlet channel 402 (not shown) exits through exit channel 405.Moreover, while cellular components of blood 501 migrate to the center(as explained below in connection with FIG. 7), cell-depleted (orcell-free) fractions of blood 501 such as plasma 502 and 503 combinewith sheath fluid 504 and 505 to exit extraction channel 400 throughexit channels 404 and 406, respectively.

It will be understood by persons skilled in the art that a membranelessseparator as described herein is not intended to, nor could it, offersufficient discrimination between the substances it is intended toremove and those it is intended to leave behind. Accordingly, forexample, membraneless separators as described above will only functionby themselves in the exceptional circumstance that all the components ofplasma are to be removed. For example, a membraneless separator may beused alone when the removal of plasma, usually not in its entirety butwithout discrimination among its components, is to be removed, and thecellular components of blood are to be left behind.

In all other circumstances, according to the principles of the presentinvention, a membraneless separator will operate in conjunction with asecondary separator that receives the sheath fluid and, optionally, acell-depleted (or cell-free) part of the bloodstream. For example, toprevent the removal of macromolecules, the secondary separator can beused to generate a stream rich in macromolecules and free of smallmetabolite molecules and middle molecules that is recycled in sheathfluid to the membraneless separator. Thus, according to the invention,the secondary separator regulates the operation of the membranelessseparator through the composition of the recycle stream that it returnsto the inlets for sheath fluid of the membraneless separator (as shownin FIG. 6 and described in greater detail below). It should beunderstood that the secondary separator may incorporate a variety ofmeans to remove solutes whose extraction removal from the circulation(i.e., the recycle stream) is desired, and that the invention is notlimited in this manner.

One substance whose transport (i.e., removal from blood being processed)is typically undesirable is albumin. In each pass through an exchangedevice according to the invention, for example, albumin would be removedat more than ¼th the rate of small solutes, and albumin (which isconfined to the blood space of an animal) would undergo perhaps 10 timesas many passes as would urea which is distributed throughout the totalbody water reservoir. Thus, the fractional removal of albumin, eventhough its inherent diffusivity is smaller, would exceed the fractionalremoval of urea. According to the principles of the present invention,therefore, a secondary separator (e.g., a membrane device that permitsextraction of urea and water but not albumin) may be used to recyclealbumin to the blood. In particular, the sheath fluid received from therecycle stream will be depleted of urea and water, but will be rich inalbumin. Thus, the composition of this stream will recruit the furtherextraction of urea and water but will not recruit further extraction ofalbumin, given that the difference in albumin concentration between theblood being processed and the sheath fluid will have disappeared.

It will be understood that an important specification of how themembraneless separator operates is the difference between the inlet flowrate and the outlet flow rate of the sheath fluid. For example, whenthese flows are equal and urea and water are removed by the secondaryseparator, there will be, at first, an insufficient transfer of waterfrom blood to the sheath fluid to keep up with water removal in thesecondary separator. Thus the concentration of proteins, includingalbumin, will rise in the recycle stream. When this concentration hasreached a sufficiently high level, water transfer will be enhanced by adifference in protein osmotic (oncotic) pressure between the blood andthe sheath fluid. Thus, the membraneless separator will balance itsperformance to that of the secondary separator. On the other hand, ifthe rate of withdrawal of sheath fluid is greater than its rate ofsupply, sufficient water may be sent to the secondary separator to keepup with its rate of water removal, but protein concentration will riseagain until a concentration difference exists in the membranelessseparator between the sheath fluid and the blood, causing a diffusion ofprotein back into the bloodstream. Once again, the membranelessseparator will balance its performance to that of the secondaryseparator.

For example, when the principal goal of the treatment is the removal ofhighly diffusible (in general, low molecular weight) molecules, assuminga flow of 20 mL/min flow, the contact area in the membraneless separatorwill be in the range about 17 to 71 cm². When the principal goal of thetreatment is the discriminating removal of slowly diffusible molecules(e.g., proteins and especially immunoglobulins), the contact area in themembraneless separator will be larger, in the range of approximately1,700 to 7,100 cm² (assuming a flow of 20 mL/min), and the secondaryseparator will be configured to remove these molecules and to recyclesmaller molecules (e.g. albumin) (unless their simultaneous removal isdesired). In another aspect of the invention, one can plasmapherese andthen remove IgG's and return the remainder of the fluid to thebloodstream.

FIG. 6 shows a simplified block diagram of a system 600 includingmembraneless separator 602 and secondary separator 604 in accordancewith the principles of the present invention. Although not shown indetail, it will be understood that membraneless separator 602 may besimilar to those separators shown in FIGS. 3 and 4 and described above,for example.

According to the principles of the present invention, blood that is toundergo processing is provided to (and removed from) membranelessseparator 602. Meanwhile, sheathing fluid that is recycled by secondaryseparator 604 is also provided to (and removed from) membranelessseparator 602. As also shown in FIG. 6, whenever secondary separator 604transfers solutes to a second fluid (e.g., dialysate), fresh dialysateconnection 606 and waste dialysate connection 608 may be used forproviding fresh and waste dialysate streams, respectively. It will beunderstood that shunting of fresh fluid directly to the blood stream, asrepresented by dashed line 610, is also a possible but not mandatorymaneuver. In general, FIG. 6 makes the role of membraneless separator602 clear: to equilibrate solutes of interest with the sheathing fluidwithout transfer of cells.

It will be understood that secondary separator 604 may use any of manyavailable separation principles known to those skilled in the art,including ultrafiltration, sorption using a wide range of sorbentstargeted to particular small and large molecules, chemical reaction, andprecipitation. Plasma diafiltration (a variant of hemodiafiltration),for example, may also be used according to the principles of the presentinvention. The following international publications which refer tohemodiafilters are incorporated by reference herein: WO 02/062454(Application No. PCT/US02/03741), WO 02/45813 (Application No.PCT/US01/47211), and WO 02/36246 (Application No. PCT/US01/45369).According to additional embodiments of the present invention, moreover,when low-molecular weight solutes are to be removed by plasmadiafiltration, a stream of sterile buffer is added to the blood to allowa greater volume of fluid, with its accompanying small molecules, topass through the diafiltration membrane. In conventional diafiltration,this volume may be added before or after the diafilter. In thisinvention, however, it is advantageous to add it either to thebloodstream or the recycle fluid from the secondary separator 604, whichis the primary source of sheath fluid.

A more detailed view of a system 700 which includes membranelessseparator 702 and secondary separator 704 in accordance with theprinciples of the present invention is shown in FIG. 7. As shown in FIG.7, separator 702 includes extraction channel 706, inlet channels 708,710 and 712 and exit channels 714, 716 and 718.

According to the principles of the present invention, system 700 alsoincludes blood supply 720, and a plurality of pumps 722, 724 and 726(which may be either manually or automatically operated, such as byusing detection and regulation techniques described below). As shown inFIG. 7, blood supply 720 provides blood to be processed to membranelessseparator 702 through blood inlet channel 710. It will be understoodthat blood supply 720 may be a living person or other animal, forexample, or may be a blood reservoir. Blood withdrawal pump 722,meanwhile, is responsible for removing blood from separator 702 throughblood exit channel 716.

As illustrated by FIG. 7, the flow of sheath fluid (or extractor fluid)into separator 702, through sheath inlet channels 708 and 712, iscontrolled by sheath fluid injection pump 724 (which preferably providessheath fluid in equal parts to channels 708 and 712). The flow of sheathfluid out of separator 702, through sheath exit channels 714 and 718,meanwhile, is controlled by sheath fluid withdrawal pump 726 (whichpreferably draws equal amounts of sheath fluid out of channels 714 and718). According to preferred embodiments of the present invention, pump724 is a two-chamber pump that provides sheath fluid at equal velocities(and with substantially similar composition) to both inlet channels 708and 712, while pump 726 is a two-chamber pump that removes sheath fluidfrom exit channels 714 and 718 at equal velocities. Moreover, it is alsocontemplated that pump 724 be replaced by two pumps (not shown) forseparately providing sheath fluid to inlet channels 708 and 712, inwhich case the composition of the sheath fluid entering inlet channel708 may be substantially similar to, or different from, the sheath fluidentering inlet channel 712. Similarly, two pumps (not shown) can be usedin place of pump 726 for the purpose of separately withdrawing sheathfluid from exit channels 714 and 718. It is also contemplated that, inother embodiments of the present invention, sheath fluid enteringthrough inlet channel 708 and exiting through exit channel 714 flows ata different velocity than the sheath fluid entering through inletchannel 712 and exiting through exit channel 718. The invention is notlimited by the particular usage of pumps or sheath velocities describedherein in connection with the description of FIG. 7.

As explained above, a membraneless separator according to the inventionalso needs two or more diverters to operate. (Four may be used when truesheathing is achieved, i.e. in sheath-blood-sheath configuration. Ingeneral, if n is the number of layers and there are inlet and outletdiverters, there can be 2(n−1) diverters.) Thus, according to theprinciples of the present invention, a first diverter 726 is formed froma portion of sheath exit channel 714 and a portion of blood exit channel716. Moreover, a second diverter 728 is formed using a portion of bloodexit channel 716 and a portion of sheath exit channel 718. It will beunderstood that, in embodiments of the present invention using more thantwo layers of sheath fluid, addition diverters will be used.

In certain preferred embodiments of the invention, the sheath fluidprovided to separator 702 (from separator 704 and/or optional sheathfluid reservoir 730) by sheath fluid injection pump 724 occupiesapproximately ⅔ of the cross-section of extraction channel 706, withblood from blood supply 720 in the middle ⅓. In this manner, each halfof the blood layer in extraction channel 706 is primarily affected byone of the sheathing layers, and the sheathing layers are traveling atan average velocity that is approximately half that of the blood (eventhough the interfacial velocities of the blood and sheathing fluids areequal). Thus, the volume of blood and the volume of sheathing fluid thatpass through the unit in a given period of time are approximately equal.Although the invention is not limited in this manner, it should be notedthat, in the configurations described here, efficiency drops when thevolumetric flows of the two fluids (i.e., blood and sheath fluid) arevery different from each other.

In order to cause the separation (or skimming) of all or part of thecell-depleted component of the blood being processed, according tovarious embodiments of the present invention, the inlet and exit flowsof the sheath fluid are controlled (via pumps 724 and 726, respectively)such that more sheath fluid is withdrawn from separator 702 than isprovided thereto. For example, it is possible to skim 10% of the bloodflow by running sheath fluid withdrawal pump 726 at a rate that is 10%higher than the rate of sheath fluid injection pump 724. It will beappreciated that, when this is done, the blood efflux rate is determinedand need not be controlled, as it should naturally have an outflow thatis 90% of the inflow.

As explained above, when indiscriminate plasma removal is not desired,the plasma that is skimmed from the blood using membraneless separator702 is processed by secondary separator 704, which regulates theoperation of separator 702 through the composition of the recycle streamthat it returns to sheath inlets channels 708 and 712 (i.e., a recyclestream is used to limit transport of blood components for whichextraction is not desirable). According to the principles of the presentinvention, a substantial benefit arises because secondary separator 704,whether membraneless or not, is able to achieve high filtrationvelocities due to the fact that concentration polarization is limited toproteins and does not involve cells. Moreover, because cells areretained in the membraneless separator 702, they would see artificialmaterial only on its conduit surfaces, not on its liquid-liquid contactarea, with the result being a reduction in bioincompatibilities and areduced (or eliminated) need for anticoagulation. Additionally, becausethe primary transport surface in the system is intrinsicallynon-fouling, a major deterrent to long-term or continuous operation isremoved, opening the possibility of a wearable system with therecognized benefits of prolonged, slow exchange.

It should be understood that any operation of membraneless separator 702that allows the sheath exit flows to be larger than the correspondinginlet values will induce a convective flow from the blood stream, overand above the diffusive flow. In order to prevent such a convective flowfrom carrying blood cells with it (as would be the case if thedistribution of cells in the blood stream was uniform), it is importantthat cellular components of the blood have migrated to the center of theblood stream in order to permit significant plasma skimming. As shouldbe appreciated by those skilled in the art, centripetal drift of cellsoccurs under a variety of flow regimes. According to the invention,therefore, various flow conditions can be used that cause blood cells tomove away from the blood-liquid interface. For example, when blood flowsin a tube below a wall shear rate (measured as the blood-flow velocitygradient perpendicular to the tube wall) of about 100 reciprocalseconds, this shear rate causes cellular components to migrate thecenter and leave the sheath as cell-free, essentially pure plasma. (SeeGoldsmith, H. L. and Spain, S., Margination of leukocytes in blood flowthrough small tubes, Microvasc. Res. 1984 March; 27(2):204-22.)

It will be appreciated that long-term stability is necessary forsatisfactory operation of the microfluidic devices described herein. Forexample, it is desirable to prevent inappropriate splitting of an exitstream which, uncorrected, could result either in loss of cells orunintended infusion of sheathing solution into the bloodstream.Moreover, the presence of blood cells in the sheath, or extractor fluidmay also be undesirable. According to another aspect of the presentinvention, therefore, on-board electronics and photonics (not shown),which are common features of chip-based microfluidic devices, may beused. In particular, such electronics or photonics could be used toregulate system 700 (i.e., to introduce flow changes) with anelectrically activated device (e.g., a piezoelectric valve) that ismounted on the same plate, or “chip,” on which separator 702 is located.

According to one embodiment of the invention, for example, very lowconcentrations of cells would be permitted and monitored (e.g., beforeor after the sheath fluid being provided to secondary separator 704) byusing any suitable detector, such as a photo detector. Anultramicroscope (a light-scattering device that is particularlysensitive in showing the presence of dilute particles) is one example ofa photo detector which can be used. Based on this monitoring, flowcorrections that would provide the system with long-term stability canbe made which include, for example, adjusting the blood-sheath fluidinterface. In particular, by adjusting the flows to separator 702 toreposition the interface, desired components can be retained in theblood. For example, when an excessive number of blood cells is present,the flow of blood could be decreased (or the flow of extractor fluidincreased) in order to shift the blood-sheath fluid interfaceaccordingly.

Additionally, according to another aspect of the invention, on-boardelectronics can be used to protect against the type of flow imbalancesthat might cause large blood losses in one direction or massivehypervolemia in the other direction, which are naturally prevented whena membrane is present but which may occur in a membraneless device. Itwill be understood by those skilled in the art this type of detectionand regulation may also be used in conjunction with the otherembodiments of the present invention described above.

As explained above, in all membraneless contact configurations, thefluids (e.g., blood and sheath fluid) must flow in the same directionand at the same rate wherever they are in contact. In particular, anydiscrepancy in magnitude or direction of the two flows, wherever theymeet, must disrupt the blood-fluid interface and induce undesirableadvection. Moreover, since the fluids must flow in the same direction,the most that can be accomplished in one membraneless unit according tothe invention is the equilibration of the sheath and blood streams(which, according to Loschmidt's formula provided above, means that ifthe sheathing fluid is flowed at the same rate as blood, the extractionE of a solute cannot exceed ½). In other words, if the two flows areequal, at most half of any solute can be transferred. Moreover, whilegreater flows permit larger fractions, E, of a solute to be removed,they require higher circulation rates to the secondary separator andthus force processing of solutes at lower concentrations, which isgenerally undesirable. Therefore, it is generally desirable for theseflows to be nearly equal, within at least a factor of 2 or 3.

This limitation on extraction can be largely overcome, however, by theconfigurations shown in FIGS. 8 and 9 and described below which achievethe effect of opposing flows (counterflow) by the juxtapositions ofmodules. In particular, low extraction efficiency can be overcome bymore sophisticated layouts of a microfluidic system such that flows areconcurrent in each unit of the system, but the overall flow approachescountercurrency in pattern and efficiency.

According to the invention, subdivision of a given, desired contact areainto n units each connected to the other in a countercurrent manner,even though the flow within them is concurrent, is used to allowextraction efficiency to rise. Thus, if an area were divided into fourunits, for example, and each had an extraction efficiency of 50%, thecomposite unit would have an efficiency of 0.8 or 80%.

FIG. 8 shows the configuration of a system 800 according to theinvention in which the total area of contact is partitioned into threesub-units 802, 804 and 806 (i.e., n=3). In operation, blood to beprocessed is first provided to sub-unit 802, then passes throughsub-unit 804, and finally, exits out of sub-unit 806. The sheath fluidto be used in system 800, on the other hand, is first provided tosub-unit 806 (at this point, the sheath fluid has no blood components).The sheath fluid exiting sub-unit 806 is next provided to sub-unit 804,and after exiting sub-unit 804, is provided to sub-unit 802. Thus,assuming each unit has an extraction efficiency of 50%, the overallextraction efficiency of the composite unit, E_(O), is equal to 0.75 or75%. Accordingly, it becomes possible, at equal flows, to remove 75%rather than only 50% of the solute of interest. In will be understoodthat the extraction efficiency approaches 1.0 or 100% as the number ofsmall units approaches infinity. Persons skilled in the art willappreciate that, although not shown, the sheath fluid exiting sub-unit802 may be provided to a secondary separator as described above.Moreover, while three sub-units 802, 804 and 806 are shown in FIG. 8, itwill be understood that any number of sub-units (e.g., 2, 4, 5, etc.)may be used in system 800, all of which may be easily introduced on amaster chip fabricated according to well known techniques for thegeneral fabrication of microfluidic devices.

FIG. 9 shows another example of a system 900 using sub-units accordingto the principles of the present invention. In particular, FIG. 9 showstwo flow patterns 902 and 904 that would be superimposed on each otherin a single cartridge. For example, the top could represent blood, whilethe bottom could represent an extractor fluid (e.g., dialysate). Asshown in FIG. 9, sheath fluid flows through sub-unit 906 prior toflowing through sub-unit 908. In this manner, with sufficient contactarea, the fraction of material in the blood layer that is extracted willbe equal to ⅔ or 67%.

Persons skilled in the art will appreciate that many differentfabrication techniques can be used in accordance with the principles ofthe present invention. In recent years, controlled fluid movement andtransport among fluids has been achieved in very small channels and atvery low rates of flow largely for the purpose of assaying the contentsof a minute fluid sample in order to determine, for example, thecatabolite concentration in the blood. These devices have been enabledby recently developed microfabrication methodologies. The Holy Grail hasbeen the development of a “Lab on a Chip,” in which several sequentialanalytical processes are conducted on a single chip that may be, forexample, one square centimeter in area. Transport of a chemical orbiochemical sample from one process to another and on and off the chipitself requires fluid handling capabilities, and thus, this enablingtechnology is commonly called “microfluidics.” Microfluidics isessential for nearly all on-chip applications. The synthesis ofchemicals in microfluidic geometries is an application that is perhapscloser in concept to the scope of this disclosure because of the need toprocess a relatively larger amount of fluid. Synthesis includes,perforce, the separations needed between the steps of a chemicalreaction sequence. While the aims of synthesizers differ from ours, andembrace some issues that we do not now see as pertinent, all of thiswork, reported and emergent, is of interest. Specifically, the presentinvention embraces some of the fabrication techniques and experimentalmethods developed for the fabrication and characterization ofmicrofluidic device structures, to define upwardly scalable transport toand from blood.

According to the invention, moreover, microchannel structures for flowexperiments may be formed by a rapid-prototyping technique. For example,the required structures may be realized in PDMS (silicone) resin byreplica-molding from master structures created in thick negative photoresist (SU-8) by optical lithography. Commercially available, standardgrade mixtures of EPON SU-8 photo resist, SU-8-5 (52% solids), SU-8-25(63% solids), SU-8 50 (69% solids) and SU-8 100 (73% solids), forexample, may be spun onto Si wafer substrates at a speed of rotationthat depended on the film thickness needed, yielding films that were 10to 300 μm thick. For example, SU-8 50 spun at 1100 rpm yields a 100 μmfilm. Prior to exposure, moreover, the spun layer is preferably baked ona precisely leveled hot plate at 95° C. for a time that is dictated bythe film thickness (ranging from minutes to hours). These samples arethen allowed to cool before further processing. Post-bake exposure,meanwhile, can be done using a direct laser writing system. Thephotolithographic setup consists of an Ar-ion laser (wavelength λ=350nm), focusing optics, and a computer controlled sample stage. Themovement of the stage along all three axes (x, y, z) is achieved bystepping motors. Desired master patterns were created by translating thesamples underneath the focused laser beam to expose the outline, andthen scanning across the interior so that the intended micro channel wasfully exposed. Dynamical focus correction or the sample tilt withrespect to the scanning laser beam was the done by on-the-flyadjustments of the distance between the focusing lens and the samplestage. In a preferred embodiment, this exposure is carried out at 95° C.for 15 min. Development, meanwhile, can be carried out in a commercialSU8 developer, again for a time based on film thickness (with the samplebeing lightly stirred during development). Patterns created in SU-8,meanwhile, are used as molding masters for replication in PDMS. PDMS isprepared from a mixture of PDMS precursor and curing agent (Sylgard 184kit, Dow Corning) in a 10:1 ratio by weight. Before curing, the mixtureis placed in vacuum to evacuate bubbles formed during mixing. It is thenpoured over the SU-8 master, which had been previously coated with athin layer (−50 nm) of chromium to improve the release of the PDMScasting, after curing. Curing is done at 70° C. for approximately twelvehours. Once the SU-8 film is spun, pre-baked and cooled as describedabove, a Karl Zeiss MJP3P Contact Mask Aligner can be used for exposure,together with standard chromium masks or transparency masks depending onthe resolution required. The films are then post-baked, and developed inthe manner outlined in the previous section. The same pattern transfertechnique is used to produce PDMS replicas.

It is apparent to those skilled in the art that many advantages may beprovided in the various embodiments of the present invention describedabove. For example, the devices, systems and methods according to theprinciples of the present invention are capable of diffusing variousblood components having different sizes, including ‘small’ molecules,‘middle’ molecules, macromolecules, macromolecular aggregates, andcells, from a blood sample to the extractor fluid. This ability isparticularly important considering the fact that different treatmentsrequire the removal of different sized particles. For example, indialysis, one may desire to remove molecules of low molecular weight,while in the treatment of acute liver failure, both small andintermediate-sized molecules are to be removed. In therapeuticapheresis, meanwhile, one generally wishes to remove selected proteinmacromolecules (e.g., immunoglobulins), while in the treatments forfulminating sepsis, it is toxins of intermediate molecular weight thatone generally desires to remove. On the other hand, in proposedanti-viral treatments, one wishes to remove free viral particles, whilein the treatment of congestive heart failure, one simply wishes toremove water.

It should also be apparent that a device or system according to theinvention may be used to process the blood of a single individual forthe purpose of treating any of a large number of disease states. Forexample, therapies according to the invention may be used in thetreatment of acute renal failure, acute liver failure, high antibodylevels in myasthenia gravis and other autoimmune diseases. Additionaluses include, for example, the removal by either precipitation orsorption of LDL in homozygous hyperlipidemia, in addition to the removalof malignant sepsis or fluid in cases of congestive heart failure, forexample. The invention may also be used to aid in the reduction of viralburdens in AIDS patients, as well as for treatment of patients requiringother types of blood purification. Patients with diabetes, patients thathave suffered a drug overdose, patients that have ingested a poison,patients suffering from renal failure, patients suffering from acute orchronic liver failure, or patients that have Myasthenia gravis, lupuserythematosis, or another autoimmune disease may also benefit from thedevices and systems of the present invention. For example, while anexchange device according to the invention is not a cure for diabetes,it can be useful in the amelioration one or more symptoms of diabetes.Moreover, the device or system of the invention could be useful inclearing the blood of IgG molecules or other molecules, which arecausative of an autoimmunity disorder. Additionally, the device orsystem of the invention can be used in acute dialysis or for extendeddialysis. There is an advantage to a system that easily separates plasmafrom cells, that permits another system to dehydrate (remove water from)the plasma in the absence of cells, and then let the transformed(extracted, reacted, etc.) plasma return. One skilled in the art willalso appreciate that patients (or animals, in the case of veterinary useof the present invention) suffering from disorders, diseases andsyndromes not listed herein may nonetheless be included in the patientpool intended for the device and system according to the invention.

Additionally, because the membraneless devices and systems describedabove have a small need for supporting machinery, and may be expected tobe much smaller, to avoid high cell concentrations and membrane contact,and to operate throughout at low rates of shear, they are especiallycompatible with cognate processes. In one embodiment, a wearable (or atleast portable) system according to the invention can run between 20 and24 hours per day at a flow rate of about 20 cc/min, for example. Thepatient could then have, for example, 4-5 hours each day without thedevice in place which could be used for personal hygiene (e.g., showersor baths), sports activities, or other activities not amenable to thesmall system being worn or used. The dialyzer should always be in placeand may require protection from aquatic environments. By accepting theconstant presence of the small device, a patient avoids the pain, risk,and treatment-limiting issues associated with inserting and removingneedles or other blood access connectors. The patient is able to hook upand detach to the device at will and painlessly, however he/she wouldhave to carry the core device with him/her. In one embodiment, the dailyor nocturnal dialysis may require active interference with thecirculation. The invention thus addresses a problem recognized by thedialysis community (i.e., the negative side effects such as physicalexhaustion, thirst, etc. associated with an episodic dialysis schedule),for which daily or nocturnal hemodialysis is not always a sufficientalternative. In particular, the invention described herein allows thepatient to move about in a normal manner (e.g., go to work, school,home, etc.) while being subject to ongoing dialysis.

In addition to the treatment of various disease states, a device orsystem according to the invention may also be used for extracting bloodcomponents that are useful in treating others, as well as for purposesof studying the processes by which molecules and cells segregate anddiffuse in blood. For example, it is known to those skilled in the artthat diffusion of individual molecular species in blood may not occurindependently and may not depend on size in the simple manner dictatedby the Stokes-Einstein equation. Moreover, many solutes may partitioninto multiple forms: free, in complexes, bound to plasma protein, boundto cell-surface moieties, or as intracellular solutes. Relative to therate of diffusion of the solute, its different forms may or may not bein local equilibrium. These phenomena are likely obscured when amembrane is present because it slows and controls overall transferrates. Therefore, a membraneless device or system according to theinvention can be a useful scientific tool to study these phenomena and asystem in which rates are raised enough that partitioning may set limitson how much and how quickly a solute can be removed. A particularexample is bilirubin bound to albumin. Another example is inorganicphosphorous which exists as partially ionized salts, as two anionicforms in plasma and in several intracellular forms.

A Membraneless Artificial Kidney Device Comprising A Flush Port—In oneaspect of the invention, the membraneless device further comprises aflush port. In a membraneless artificial kidney, it is difficult to keepall blood cells from entering the sheath fluid. If the cells do enterthe sheath fluid, it is likely that they will not return to thebloodstream before they have reached a prohibitive concentration, andthere is no other method of egress. The invention provides for a flushport in the membraneless device. A sterile fluid is injected into thesheath fluid, flushing it and its contents into the bloodstream. Thesheath fluid is thus replaced by clear, fresh fluid. This process may beconducted intermittently and at such intervals as not significantly toincrease the overall volume of liquid to be removed from the blood.

Direct contact between uremic blood and a fluid capable of receivinguremic toxins is possible. Such contact by itself is, however, notbeneficial because it depends on diffusion coefficients in blood toselect the molecules that are removed. This selection is inadequate andwould result in the exhaustion of a patient's albumin pool before usefulreduction in the urea pool was achieved. Direct contact that isaccomplished by sandwiching blood between two layers of a “sheathing”fluid, followed by diafiltration of the sheathing fluid throughconventional membranes and recirculation of the sheathing fluid, ispossible. This adaptation of membraneless transport of molecules fromblood eliminates almost all contact of blood with solid artificialsurfaces and the subsequent diafiltration and recirculation of thesheathing fluid allows precise control of what is removed from thesystem. Slightly hyperosmotic protein is carried back by therecirculating sheathing fluid. Only solutes and water that pass thediafilter, which operates on a cell-free fluid, are able to leave thesystem. The system depends strongly on the ability to keep cells out ofthe sheathing fluid. A quantitative design of a wearable dialyzer basedon a circulating sheathing fluid is presented.

All biological transport between phases uses membranes. This is true atthe submicroscale where each organelle of a cell possesses a membrane,as does the cell itself. It is true at all higher scales where weencounter the vascular endothelium, the alveolar membrane, theintestinal wall, and many epithelial surfaces. Faced with the prevalenceof membranes throughout biological systems, one must ask the veryserious question, why would one even attempt mass transfer from onefluid to another without a membrane? The answer to this question lies inthe imperfections of man-made membranes: (1) A typical dialysis membraneis at least 1000 times thicker than its natural counterpart. (2) Itsinterface foments a largely inappropriate set of chemical reactions withblood components. (3) In long-term use it becomes fouled—clogged withmolecular aggregates that impede transport.

A dialysis system is wearable and works as a dialyzer through whichblood would flow constantly is provided by the invention. [1-7]. Such adevice needs to be small, which means that its exchange rates betweenblood and dialysate need to be fast. Its blood-wetted surfaces should behighly biocompatible and should not require heparinization of the bloodflowing continuously past them. Since the blood-wetted surface isessentially permanent, it should be much more resistant to fouling thanany artificial membrane now known. These qualities could be obtained ifit were possible to have direct, liquid-liquid contact of blood withdialysate. Membranes are important in systems because they do thefollowing: (1) Membranes select—so that some molecules pass through themand others do not. In biological systems, transport without molecularselectivity at some point in the transport path is essentially useless.(2) Membranes offer a mechanical barrier that prevents gross mixing oftwo otherwise miscible fluids, and they permit the use of a pressuredifference to extract water. (3) Membranes define the boundaries ofdifferent compartments.

The invention provides for a system that offers the advantages of bothmembrane-moderated and membraneless transport. In one aspect, theinvention encompasses such a system devised as a wearable hemodialysissystem.

Microfluidic-Membraneless Transport

It has been know for many years that one fluid could be flowed besideanother without convective mixing [8,9]. However, the concept becameuseful only with the advent of reliable means of microfabricating thinfluid channels, usually through the use of photolithographic andmicromachining techniques that were developed first for manufacturinglarge-scale, integrated electronic devices on silicon chips. In thesystem considered here, the flow of three liquid layers are examined,each—nominally—100 μm thick. To a very good approximation the outer,sheathing layers will have a mean speed that is half that of the center(blood) layer (FIG. 10), so that, overall, the two liquids will have thesame flow rate. Systems with different flow rates are possible but aregenerally less desirable. It is important to notice that the flows indirect contact must be in the same direction, as shown in FIG. 10. Anattempt at counter- or cross-flow would lead to gross mixing of thestreams.

How quickly will two streams, contacted in this manner, come toequilibrium?Boltzmann's result is a complex formula but can berepresented graphically as in FIG. 11. For small molecules, withdiffusion coefficients, D, equal to about 10⁻⁵ cm²/sec, such as ions,sugars, and urea in blood, 90% equilibration will occur in about 2.1sec, a remarkably short time that reflects the thinness of the layersand the absence of a membrane. For larger molecules, e.g. albumin (Dequal to ˜5·10⁻⁷ cm²/sec) 95% equilibration will occur in about 42 sec.This difference might seem large enough to afford discrimination betweensmall molecules and proteins, but it is not. (For the same exposuretime, which is the condition to be met in any one device and flow, thebest selectivity between small and large molecules depends on the squareroot of their diffusion coefficients, 1/4.5 for the case consideredhere. In most practical situations, the selectivity is, in fact, worse.In a simple, direct contact blood treatment, the plasma albumin poolwould be completely removed long before there was reasonable depletionof urea in body water [13].) A membraneless interface, by itself, isindiscriminate.

A Recirculating Sheath

If the blood-contacting fluid is not dialyzing fluid but rather acontinuously circulating fluid that is dialyzed in a small, conventionaldialyzer, one has the system shown in FIG. 12. The governing principleof this system is that both the blood and the sheath fluid musttransport what the conventional dialyzer permits them to transport—nomore, no less. Any solute not removed by the conventional dialyzeraccumulates in the sheath fluid and returns to the blood contactor. Thesame principle applies if the device that extracts material from thesheath fluid is a hemodiafilter, absorber, or chemical reactor.

The extraction device also controls volume transport, although theresultant situation in the blood contactor is a little harder tointerpret. After a short startup period, the protein concentration inthe sheath fluid exiting the blood-sheath contactor will come close tothe protein concentration in blood. If water is ultrafiltered from theconventional dialyzer, the sheath fluid leaving the conventionaldialyzer and returning to the blood-sheath fluid contactor will have ahigher protein concentration that that in blood, and will thus behyperosmotic. Water then passes from blood into the sheath fluidosmotically, without a hydrostatic pressure difference, at a rate setprecisely by the ultrafiltration rate of the conventional dialyzer. Howwater is removed from the device that extracts materials from the sheathfluid is immaterial; only the amount removed is consequential. Thus, inthis system, control of transport in the conventional dialyzer definesprecisely both volume and solute transport out of the blood layer. Onemay, however, well ask, if there is a conventional dialyzer in thesystem, what overall advantage has been gained? There are at least threeadvantages: (1) Whole blood does not contact an artificial membrane; theblood interface is a pair of moving liquids. The interface should behighly biocompatible and cannot be fouled; anything that might depositat the interface would be swept away. Because volume transport isosmotic, there is no tendency to draw cells to the interface. (2)Transport is very rapid. While one might ask whether transport in theconventional dialyzer is not now the limiting factor, the absence ofcells in this device allows for higher rates of shear andultrafiltration than would be possible if whole blood were present. (3)The shear stresses imparted to the blood layer are extremely low. Thehighest shear stress in the system is low and occurs in the sheath fluidwhere it contacts the wall.

Preventing Cells from Entering the Sheath Fluid

The systems presented here are effective if blood cells, particularlyerythrocytes whose conservation is a recognized criterion of gooddialysis, do not migrate into the sheath fluid. It is important tounderstand how stringent this requirement must be, although some ways ofrelaxing it will be discussed below. If 100 L of patient blood aredialyzed per week, and cell loss is kept at 1:10,000, the volumetricblood loss per week would be 10 ml, probably less than lossesencountered in current therapies, and probably acceptable. Cellsentering the sheath fluid must either remain there or return to thebloodstream, which they are unlikely to do, unaided. In certainembodiments of the system of the invention, there is no place for thesmall number of cells that may migrate into the sheath fluid to escape.FIG. 12 shows a flush port that allows for the intermittent injection ofa small volume of sterile saline into the sheath fluid. Such aninjection would force return of entrapped cells to the circulation.Because sheath fluid volume is less than 5 ml, displacing the entiresheath fluid volume into the bloodstream is equivalent to the volumeremoved in about 2 min of ultrafiltration.

The general tendency for cells to migrate to the center of a flowingstream is well documented [14-16]. In addition to this generalphenomenon, it has been shown that when shear rates are low enough toallow rouleaux formation, the migration is even more pronounced [17].Measurements of cell migration in a prototype membraneless deviceconfirm that cells migrate well to the center of a smooth, steadilyoperated flow channel [13].

How Would A Real Ambulatory Dialyzer Perform?

An ambulatory dialyzer, through which blood is flowing at all times,need not be dialyzing at all times. In one embodiment of the invention,such a device might have a continuous blood flow of 40 ml/min and beattached to a source of dialysate 50% of the time, 84 hr/wk. Thus, justover 200 L of patient blood would be dialyzed per week. Because theflows in this device are concurrent and, in present designs,approximately equal, the maximum urea clearance will be relatively low,approximately 18 ml/min, leading to an estimated Kt/V of about 1.8. Inone aspect, one can assume that volume transport might reach 15 L/wk,which could be accomplished during dialysis (3 ml/min) or continuously(1.5 ml/min), since dialysate is not required to produce ultrafiltratethrough the conventional dialyzer unit. The rapid equilibration timesfor small molecules cited above lead to en-face contact areas betweenblood and sheath fluid of only about 50 cm^(2[13]). (The actual contactarea, because the sheath fluid contacts both sides of the bloodstream,is about 100 cm².) The change in pressure from entrance to exit of theblood-sheath contactor is about 5 mmHg.

It is important to recall that blood flow is continuous in this systemand that the starting and stopping of actual dialysis requires only thenot-necessarily-sterile attachment and detachment, respectively, ofdialysate leads. A patient is given a dialysis prescription in the formof the required number of hours per week of dialysis, and is leftlargely free to decide when to be attached to, or detached from, asource of dialysate. Most patients would accomplish the bulk of theirdialysis overnight. The conventional dialyzer will foul, although moreslowly because it operates in a cell-free, high-shear environment. It ispossible that it will need to be replaced every other day. The inventionprovides for prototypes of this dialyzer, with areas of 500 and 1000cm², for diffusive and hydraulic permeability and for the rate at whichperformance decrements. The shell for these devices looks like amini-conventional dialyzer (FIG. 13).

What Would A Real Ambulatory Dialyzer Look Like?

In one embodiment of the invention, the ambulatory dialyzer would beworn on the lower arm. The unit is comprised of five elements: (i) ablood-sheath contactor, a two-layer plate whose dimensions are about5.5×8×0.6 cm; (ii) a conventional dialyzer of FIG. 13 reconfigured tohave an elliptical or rectangular cross-section; (iii) a small,battery-operated, two-tube peristaltic pump that maintains both bloodand sheath fluid flows; (iv) an exchangeable, rechargeable battery withdimensions similar to that of the blood sheath contactor; (v) a smallcontrol module. These elements are shown in one embodiment as they mightbe worn on a patient's forearm in FIG. 14. One approach is the use oftwo catheters similar to those that are used in long-term totalparenteral nutrition.

An important concomitant of the proposed system is the ability of thepatient to secure maintenance of his device by going to a servicecenter. Some patients may be able to change the dialyzer module; othersmay wish assistance. Assistance could be provided at a walk-in serviceunit. With or without assistance, the proposed system is designed toempower patients in the management of their disease.

Other Realizations of the Membraneless Device

Plasmapheresis: The flowpath of the blood-sheath contactor appears toafford an excellent geometry for achieving plasmapheresis. Under thesecircumstances, an initial charge of sheath fluid would rapidly becomeequivalent to plasma and, during apheresis, a fraction of thecirculating sheath fluid, equivalent to plasma, would be continuouslywithdrawn. Present plasmapheresis devices require either membranecontact or the application of centrifugal force. In this application ofthe membraneless flowpath, neither would be required.

Studies of molecular transport in blood: Membraneless transport offers apotentially important, non-clinical opportunity. The movement of manymolecules through blood and, sometimes, through dialysate is poorlyunderstood. This is true of molecules distributed across extracellularand intracellular space if their performance is not trivialized becausethey are either instantly equilibrated or completely unaffected astransport occurs. It is also true for molecules that are bound toslower-moving molecules or cell surfaces and dissociate from thesehavens as transport occurs. Finally it is true of molecules that changetheir molecular shape, or charge, or degree of aggregation withconcentration. The transport of these molecules in blood is not fullyunderstood, in large part because the study of this transport oftenoccurs in the presence of a membrane whose dominant resistance obscuresanomolous intraphase transport. For example, the factors limitingtransport of phosphate, bilirubin and fatty acids in blood are not fullyunderstood. Membraneless transport emphasizes exchange within phases,not across their boundary, and can even be conducted between contiguouslayers of blood, only one of which, initially, contains the solute ofinterest.

Transport without molecular discrimination is valueless. Membranelessdialysis is, in fact, not possible, but membraneless transport coupledwith sheath dialysis is possible and probably practical. It representsan advance over current membrane systems, especially in the muchdesired, but difficult to achieve, modality of long, slow, safeambulatory renal replacement therapy.

REFERENCES

-   1. Blackshear, P. L., 2 New Concepts That Might Lead To A Wearable    Artificial-Kidney. Kidney International, 1978: p. S133-S137.-   2. Blaney, T. L., O. Lindan, and R. E. Sparks, Adsorption—A Step    Toward A Wearable Artificial Kidney. Transactions American Society    for Artificial Internal Organs, 1966. 12(APR): p. 7-   3. Henne, W., et al., Wearable Artificial-Kidney. Artificial    Organs, 1977. 1(1): p. 126-126.-   4. Kolff, W. J., et al., Towards A Wearable Artificial-Kidney.    Kidney International, 1976. 10: p. S300-S304.-   5. Saito, A., et al., Maintaining Low Concentrations Of Plasma    Beta(2)-Microglobulin Through Continuous Slow Hemofiltration.    Nephrology Dialysis Transplantation, 1995. 10: p. 52-56.-   6. Seo, S., et al., Improvement Of The Wearable Artificial-Kidney.    Artificial Organs, 1981. 5(3): p. 321-321.-   7. Vanholder, R. and S. Ringoir, Pitfalls Of Wearable    Artificial-Kidney. International Journal Of Artificial Organs, 1990.    13(11): p. 715-719.-   8. Giddings, J. C., Continuous Separation In Split-Flow Thin    (Splitt) Cells—Potential Applications To Biological-Materials.    Separation Science And Technology, 1988. 23(8-9): p. 931-943.-   9. Levin, S. and G. Tawil, Analytical Splitt Fractionation In The    Diffusion Mode Operating As A Dialysis-Like System Devoid Of    Membrane—Application To Drug-Carrying Liposomes. Analytical    Chemistry, 1993. 65(17): p. 2254-2261.-   10. Fazio, F., Artificial Kidney and Methods of Using Same, in    European Patent Register. 2002, Renal Plant Corp.: EU.-   11. Ronco, C. Microfluidic, Membrane-Free Dialysis. in American    Society of Nephrology, Annual Meeting. 2002.-   12. Wellman, P. S., Substantially Inertia Free Hemodialysis. 2004:    US.-   13. Leonard, E. F., et al., Dialysis without membranes: How and why?    Blood Purification, 2004. 22(1): p. 92-100.-   14. Moger, J., et al., Measuring red blood cell flow dynamics in a    glass capillary using Doppler optical coherence tomography and    Doppler amplitude optical coherence tomography. Journal Of    Biomedical Optics, 2004. 9(5): p. 982-994.-   15. Singh, M. and A. T. V. Ramesh, Hematocrit Dependence Of Cellular    Axial Migration And Tubular Pinch Effects In Blood-Flow Through    Glass-Capillaries. Current Science, 1990. 59(4): p. 223-226.-   16. Uijttewaal, W. S. J., E. J. Nijhof, and R. M. Heethaar, Lateral    Migration Of Blood-Cells And Microspheres In 2-Dimensional    Poiseuille Flow—A Laser-Doppler Study. Journal Of    Biomechanics, 1994. 27(1): p. 35-42.-   17. Goldsmith, H. L. and S. Spain, Margination Of Leukocytes In    Blood-Flow Through Small Tubes. Microvascular Research, 1984.    27(2): p. 204-222.-   18. Loschmidt J: Experimental—Untersuchungen uber die Diffusion von    Gasen ohne porose Scheidewande. Sitzungsber Kais Akad Wiss Wien Math    Naturwiss K1 II 1870; 61.367.-   19. Wakeham W A, Kestin J: The measurement of diffusion    coefficients, in Ho CY (ed): Transport Properties of Fluids. New    York, Hemisphere, 1988, pp 225-228.

Persons skilled in the art will also appreciate that the presentinvention can be practiced by other than the described embodiments,which are presented for purposes of illustration and not of limitation,and that the present invention is limited only by the claims thatfollow.

1-43. (canceled)
 44. A method of performing a blood treatment on apatient, the method comprising: attaching blood lines of a blood tubingset to a microfluidic channel; attaching a membrane-based filter deviceto the microfluidic channel so as to define a recirculating flowpathuninterrupted by a membrane; at a treatment time: attaching a source ofdialysate to the membrane-based filter device such that the dialysateflows on one side of a membrane of the filter device; permitting toxinsand water to pass from the microfluidic channel through the membrane ofthe filter device into the dialysate flow; and returning components ofthe patient's blood passing in the passing in the recirculating flowpathback to the patient's blood; and after the treatment time, disconnectingthe source of dialysate while leaving the blood lines attached to themicrofluidic channel.
 45. The method of claim 44, further comprising, ata second treatment time after the disconnecting, repeating the attachinga source of dialysate, the permitting, and the returning.
 46. The methodof claim 45, further comprising, after the disconnecting and before therepeating, flowing blood and fluid together in the microfluidic channel.47. The method of claim 44, wherein said permitting includessimultaneously flowing blood from the patient and fluid from themembrane-based filter device together in the microfluidic channel. 48.The method of claim 47, wherein the simultaneously flowing is such thatthe toxins and water move from the blood flow into the fluid from themembrane-based filter device without passing through any membrane. 49.The method of claim 44, wherein said components of the patient's bloodinclude blood proteins.
 50. A method of performing a blood treatment ona patient, the method comprising: co-flowing blood from the patient andrecirculating fluid from a membrane-based filter device through amicrofluidic channel such that toxins, water, and blood components aretransferred from the blood flow into the recirculating fluid flowwithout passing through a membrane, the recirculating fluid flow withthe toxins, water, and blood components exiting the microfluidic channelat an outlet thereof and returning to the membrane-based filter device;at a treatment time: connecting the membrane-based filter device to asource of dialysate; and flowing the dialysate from the source along aside of a membrane in the membrane-based filter device such that toxinsand water in the recirculating fluid flow move across the membrane intothe dialysate flow; and after the treatment time, disconnecting themembrane-based filter device from the source of dialysate.
 51. Themethod of claim 50, wherein the co-flowing blood and recirculating fluidand the flowing the dialysate occur at a same time.
 52. The method ofclaim 50, further comprising, after the disconnecting: continuing toco-flow the blood and the recirculating fluid through the microfluidicchannel.
 53. The method of claim 50, wherein the blood components in therecirculating fluid flow exiting the microfluidic channel are returnedto the microfluidic channel.
 54. The method of claim 53, wherein theco-flowing blood and recirculating fluid is such that the bloodcomponents in the exiting recirculating fluid flow are returned to theflowing blood.
 55. The method of claim 50, wherein the blood componentsinclude albumin.
 56. The method of claim 50, wherein the recirculatingfluid exiting the microfluidic channel flows through the membrane-basedfilter device on an opposite side of the membrane and is returned to themicrofluidic channel to co-flow with the blood therein.
 57. A method ofperforming a blood treatment on a patient, the method comprising:flowing blood from the patient and recirculating fluid from a secondaryprocessing device together along a microfluidic channel in a primaryprocessing device such that toxins, water, and blood components areremoved from the blood flow without passing through a membrane,conveying fluid with the removed toxins, water, and blood componentstherein from an outlet of the primary processing device to an upstreaminlet of the primary processing device via a flowpath through thesecondary processing device; and periodically using the secondaryprocessing device to eliminate water and toxins from the fluid in theflowpath through the secondary processing device.
 58. The method ofclaim 57, wherein the removed blood components at the outlet of theprimary processing device are returned to the flowing blood in themicrofluidic channel.
 59. The method of claim 57, wherein: the secondaryprocessing device is a membrane-based filter device; and water andtoxins are eliminated from the fluid flowing through the filter deviceby passing the water and toxins across a membrane into a dialysate flow.60. The method of claim 57, wherein the periodically using the secondaryprocessing device includes: connecting the secondary processing deviceto a source of dialysate; and flowing the dialysate from the sourcealong a side of a membrane in the secondary processing device such thatthe toxins and water move across the membrane into the dialysate flowwhile blood components are retained in the fluid flowing on an oppositeside of the membrane.
 61. The method of claim 57, wherein when thesecondary processing device is not used to eliminate water and toxins,the secondary processing device is not connected to a source ofdialysate.